1. Field of the Invention
The present invention relates to a method of connecting base materials in which a pair of base materials (combination of a circuit board and a semiconductor chip, combination of a semiconductor chip and a semiconductor chip, etc.) are connected via electrodes.
2. Description of the Related Art
There is a growing demand for high-density mounting of electronic components with recent progress in down-sizing and thinning of electronic instruments, wherein flip chip bonding has been adopted, by which electronic components such as semiconductor chips in a uncovered form are directly mounted on a substrate. Electrodes of semiconductor chips used for the flip chip bonding have bump electrodes formed thereon, and the bump electrodes and wirings on a circuit board are electrically connected.
Representative methods of forming metal terminals include electroplating process, electroless plating process, solder dipping process and solder printing process.
In the electroplating process, a work is placed in a plating solution, and the metal terminals are formed en bloc on electrode pads patterned by a photolithographic process, while supplying current through seed electrodes connected to the electrode pads. The process is characterized in that use of a high-resolution photoresist makes it possible to form the metal terminals having a large aspect ratio in several micrometers to several tens of micrometers pitch. Materials available for forming the metal electrodes by the electroplating process include gold, solder and so forth.
The electroless plating process makes it possible to form the electrode pads en bloc on arbitrary electrode pads. The process is characterized in causing anisotropic growth of a plated film, and disusing the photolithographic process.
In the solder dipping process, a work having electrode pads formed thereon is dipped in and pulled up from a molten low-melting-point metal mainly composed of Sn and Pb, so as to allow the low-melting point metal to wet on the electrode pads with the aid of the surface tension, and then to cool and solidify thereon so as to form the metal terminals.
In the solder printing process, a low-melting-point metal mainly composed of Sn and Pb in a paste form is coated by printing to recessed portions preliminarily formed on a metal plate so as to correspond with position of the electrode pads, the metal is allowed to re-flow so as to form spherical electrodes, and the spherical electrodes are then transferred en bloc onto the electrode pads on a work.
On the other hand, there are known various bonding methods of electronic components such as LSI, which include diffusion bonding using a molten solder, solid phase bonding of metals, and bump-to-bump bonding via an insulating adhesive film. All of these methods effect bonding while applying some stress, such as load, heat or ultrasonic wave to the bumps.
There is also a recent trial in which the surface of a semiconductor chip having the above-described metal terminals formed thereon is covered with an insulating resin having adhesiveness, the chip is placed on a circuit board so as to oppose the metal terminals of the both, and the insulating resin is then allowed to cure to thereby bond the metal terminals.
[Patent Document 1] Japanese Patent Application Laid-Open No. Hei 11-274241
[Patent Document 1] Japanese Patent Application Laid-Open No. 2001-298146
[Patent Document 1] Japanese Patent Application Laid-Open No. 2003-258034
[Patent Document 1] Japanese Patent Application Laid-Open No. 2002-252245
The above-described conventional processes, however, raised the problems as described below, when applied to formation of the metal terminals of electronic components such as LSI, and to mounting of these components.
The electroplating process needs a mask used for patterning, a photolithographic process, formation and removal processes of the seed electrodes, and is causative of a large increase in the process cost.
The electroless plating and solder dipping processes are causative of non-uniformity in height of the metal terminals as large as several micrometers to several tens of micrometers depending on geometry and area of the electrode pads, presence or absence of connection to a wiring pattern, and non-uniformity in conditions of the plating or dipping.
The solder printing process encounters limitation in micronization of the metal terminals because a paste is printed using a squeegee.
The method of covering the metal terminals with an insulating resin raises a problem in that the resin which remains between the metal terminals undesirably elevate electric resistance.
In the mounting of LSI components, a certain amount of external energies such as load and heat are required due non-uniformity in the height, and this inevitably results in some damage on the mounted LSI components.
It is therefore an object of the present invention, aimed at solving the above-described problems, to provide a method of connecting base materials capable of forming the metal terminals with uniform height and smooth surface at low costs, and of realizing low-damage mounting.